Electrical components such as connectors and electronic components such as leadframes are manufactured from copper alloys to exploit the high electrical conductivity of copper. Pure copper such as C10200 (oxygen-free copper having a minimum copper content by weight of 99.95%) has a yield strength in a spring temper of about 37 kg/mm.sup.2 (52 ksi) which is too weak for applications in which the component is subject to forces associated with insertion and removal. To increase the strength of copper, a wide array of alloying elements have been added to copper. In most cases, there is a tradeoff between the increase in yield strength achieved by the alloying addition with a resultant decrease in the electrical conductivity.
Throughout this application, alloy designations such as C10200 utilize the Unified Numbering System designations. Compositional percentages are in weight percent unless otherwise noted.
For electrical and electronic applications zirconium and mixtures of zirconium and chromium are frequently added to copper. For example, copper alloy C15100 (nominal composition 0.05-0.15% zirconium and the balance copper) has an electrical conductivity of 95% IACS (IACS stands for International Annealed Copper Standard where unalloyed copper is defined as having an electrical conductivity of 100% IACS). C15100 has a spring temper yield strength of no more than 46 kg/mm.sup.2 (66 ksi). A copper-zirconium intermetallic phase precipitates from the copper matrix as a discrete second phase following heat treatment (precipitation hardening) increasing the strength of the alloy. However, the yield strength of C15100 is still too low for the current trend to higher strength connectors and leadframes in miniaturized applications.
Higher strength is obtained by adding a mixture of chromium and zirconium to copper. C18100 (nominal composition 0.4%-1.0% chromium, 0.08%-0.2% zirconium, 0.03-0.06% magnesium and the balance copper) has an electrical conductivity of 80% IACS at a yield strength of from 47-50 kg/mm.sup.2 (67 to 72 ksi). The electrical conductivity of C18100 is acceptable, however, the yield strength is slightly lower than desired. Also, a chromium content above the maximum solid solubility of chromium in copper, about 0.65% for a copper/chromium binary alloy, leads to large second phase dispersions which contribute to a poor surface quality and non-uniform chemical etching characteristics.
For leadframes requiring high heat dissipation to prolong semiconductor device life and electrical connectors carrying high currents where ohmic heating is detrimental, it is desirable to have an electrical conductivity above about 70% IACS and a yield strength above about 56 kg/mm.sup.2 (80 ksi).
The alloy should have good stress relaxation resistance properties both at room temperature and at elevated (up to 200.degree. C.) service temperatures. When an external stress is applied to a metallic strip, the metal reacts by developing an equal and opposite internal stress. If the metal is held in a strained position, the internal stress will decrease as a function of both time and temperature. This phenomenon, called stress relaxation, occurs because of the replacement of elastic strain in the metal to plastic, or permanent strain, by microplastic flow. Copper based electrical connectors are frequently formed into spring contact members which must maintain above a threshold contact force on a mating member for prolonged times. Stress relaxation reduces the contact force to below the threshold leading to an open circuit. Copper alloys for electrical and electronic applications should, therefore, have high resistance to stress relaxation at both room and high ambient temperatures.
The minimum bend radius (MBR) determines how severe a bend may be formed in a metallic strip without "orange peeling" or fracture along the outside radius of the bend. The MBR is an important property of leadframes where the outer leads are bent at a 90.degree. angle for insertion into a printed circuit board. Connectors are also formed with bends at various angles. Bend formability, MBR/t, where t is the thickness of the metal strip, is the ratio of the minimum radius of curvature of a mandrel around which the metallic strip can be bent without failure and the thickness of the metal. ##EQU1##
An MBR/t of under about 2.5 is desired for bends made in the "good way", bend axis perpendicular to the rolling direction of the metallic strip. An MBR of under about 2.5 is desired for bends made in the "bad way", bend axis parallel to the rolling direction of the metallic strip.
In summary, a desirable copper alloy for electrical and electronic applications would have the combination of all of the following properties:
Electrical conductivity greater than 70% IACS.
Yield strength greater than 56 kg/mm.sup.2 (80 ksi).
Resistance to stress relaxation at a temperature as high as 150.degree. C.
MBR/t less than 2.5 in the "good way" and "bad way".
The copper alloy should resist oxidation and etch uniformly. The uniform etch provides sharp and smooth vertical lead walls on etched leadframes. A uniform chemical etch during precleaning also promotes good coatings by electrolytic or electroless means.
U.S. Pat. No. 4,872,048 to Akutsu et al, discloses copper alloys for leadframes. The patent discloses copper alloys containing 0.05-1% chromium, 0.005-0.3% zirconium and either 0.001-0.05% lithium or 5-60 ppm carbon. Up to about 2% of various other additions may also be present. Two disclosed examples are Alloy 21 (0.98% chromium, 0.049% zirconium, 0.026% lithium, 0.41% nickel, 0.48% tin, 0.63% titanium, 0.03% silicon, 0.13% phosphorous, balance copper) with a tensile strength of 80 kg/mm.sup.2 (114 ksi) and an electrical conductivity of 69% IACS and Alloy 75 (0.75% chromium, 0.019% zirconium, 30 ppm carbon, 0.19% cobalt, 0.22% tin, 0.69% titanium, 0.13% niobium, balance copper) with a tensile strength of 73 kg/mm.sup.2 (104 ksi) and an electrical conductivity of 63% IACS.
Great Britain Patent Specification No. 1,353,430 to Gosudarstvenny Metallov, discloses copper-chromium-zirconium alloys containing tin and titanium. Alloy 1 contains 0.5% chromium, 0.13% titanium, 0.25% tin, 0.12% zirconium, balance copper with a tensile strength of 62-67 kg/mm.sup.2 (88-95 ksi) and an electrical conductivity of 72% IACS.
Great Britain Patent Specification No. 1,549,107 to Olin Corporation, discloses copper-chromium-zirconium alloys containing niobium. Dependent on the method of processing, an alloy containing 0.55% chromium, 0.15% zirconium, 0.25% niobium and the balance copper has a yield stress of from 51-64 kg/mm.sup.2 (73-92 ksi) and an electrical conductivity of 71-83% IACS.